Optical fiber cable

ABSTRACT

An optical fiber cable of the type comprising a fiber (3) having a central core and a surrounding cladding and made for transmitting optical high-power, specifically power exceeding 1 kW. At least one of the fiber ends is provided with cooling means for optical power loss comprising a cavity with a flowing coolant (2) surrounding the envelope surface of the fiber end. Incident radiation that falls outside the fiber comes into the coolant, preferably a liquid coolant such as water, where it is at least partially absorbed. At least one of the limiting wall surfaces of the cavity is completely or partially non-absorbing for the incident radiation, while the other limiting wall surfaces are arranged to absorb such radiaton that is still present and transmitted through the flowing coolant (2). As these surfaces are in direct contact with the coolant an efficient cooling is obtained.

BACKGROUND OF THE INVENTION

The present invention relates to an optical fiber cable which comprisesa fiber having a central core and a surrounding cladding and which fibercable is made for transmitting high optical power, specifically powerexceeding 1 kW. At least one of the contact ends of the fiber hascooling means for optical power loss.

Optical fiber cables for transmitting high optical power are frequentlyused in industrial applications. Specifically they are used in cuttingand welding operations by means of high-power laser radiation, but alsoin other industrial applications such as heating, detection or workingoperations in high-temperature environments this type of optical fibercables can be used. One of the main problems in this type of high powerapplications, however, is the problem how to take care of radiation thatfalls outside the core of the fiber. The power density is normally veryhigh and specific cooling means are required in order to prevent anuncontrolled heating, especially in case of strong back scattering infor instance welding operations.

Different methods to take care of such undesired power radiation arealready known. One example is disclosed in DE 4305313, in which theradiation that falls into the cladding of the fiber is spread in aso-called mode stripper and absorbed by a metal surface. This surfacecan then be cooled from the outside of the component. A similar methodis described in SE 83.07140-7.

An optical fiber cable of the above-mentioned type is also presented inSE 93.01100-5. In said fiber cable at least one of the end surfaces ofthe fiber core is provided with a rod having a larger diameter than thecore diameter. At this end the fiber is provided with a reflectordesigned to conduct rays entering outside the fiber towards an areawhere they can be absorbed without causing any damage. In theillustrated embodiment this area is surrounded by a heat-abductingdevice with cooling fins, but it is also mentioned that water coolingmeans may be included in this area for cooling off the generated heat.Also in this case the cooling is provided from the outside of thecomponent.

A weakness in all of these methods that now have been described is thefact that the heat first must be absorbed by a metal surface and thenconducted through the metal material to the cooled surface either thissurface is cooled by means of air or by water.

SUMMARY OF THE INVENTION

The object of this invention is to provide an optical fiber cable withan improved cooling capability so that the fiber can be used fortransmitting a very high optical power without causing any damage to thefiber itself or to the housing. The invention is based upon the factthat the power (heat) is absorbed directly in a cooling medium insteadof being conducted through a metal material.

According to the invention at least one of the contact ends of thefiber, comprising the core and the surrounding cladding, are located ina cavity filled with a flowing coolant so that radiation falling outsidethe fiber is entered into and absorbed at least partially by thecoolant. The walls of the cavity comprises at least one non-absorbingsurface. The other wall surfaces could be absorbing (metal). Radiationpassing through the coolant is absorbed by said surfaces. As thesesurfaces are directly cooled by the flowing medium (liquid coolant) anyuncontrolled heating can be avoided. As the optical radiation is passingthrough the flowing coolant before reaching the metallic surface, only aminor part of the radiation is absorbed by the surface.

According to a preferred embodiment the fiber is directly in contactwith the surrounding coolant, for example water.

According to an alternative embodiment the fiber is surrounded by atransparent tube which then is directly in contact with the surroundingcoolant.

In the following the invention will be described more in detail withreference to the accompanying drawings which schematically illustratessome examples of the new optical fiber cable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principles for an optical fiber cable with direct watercooling means of the contact end of the cable,

FIG. 2 is a detailed view of the interface zone between the end portionof the fiber and the transparent "window" through which the radiation isentered into the cavity with the liquid coolant surrounding the endportion of the fiber,

FIG. 3 shows a so-called mode stripper arranged around the fiber in theliquid coolant cavity in order to transmit radiation in the cladding ofthe fiber out to the surrounding coolant,

FIG. 4 is a detailed view of the interface zone between the end portionof the fiber and the transparent window according to an alternativeembodiment in which the "window" is made as an optical disc with acentral bore which disc is sealed against the circumferential surface ofthe fiber, and

FIGS. 5a-5c shows three examples how to arrange a transparent capillarytube around the fiber in the liquid coolant cavity.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one end of a conventional optical fiber 3 having acore, for example of quartz glass, and a cladding, for example made ofglass or some polymer having a suitable refractive index.

A laser beam 1 is focused on the end surface of the fiber. Preferably aNd-YAG laser source is used which has a wave-length of 1.06 μm. Thiswave-length is suitable for optical fiber transmission. Other examplesof lasers that can be used is diode lasers, CO₂ -lasers, CO-lasers andother types of Nd-lasers.

A liquid coolant 2 is surrounding the envelope surface of the endportion of the fiber. That part 4 of the incident laser radiation thatfalls outside, the core of the fiber is entered into and absorbed, atleast partially, by the coolant. Radiation transmitted through theliquid is absorbed by the walls 8,5 enclosing the liquid. These wallsare in direct contact with the coolant so that they are cooled directlyon the surface. The rear wall 5 has an in put pipe 5a as well as anoutput pipe 5b for the liquid coolant.

The absorption in the liquid should not be too high due to the risk ofshock boiling of the liquid when it is hit by the radiation. Water is asuitable cooling medium, for simplicity, but also for the reason thatthe deep of penetration is suit able. For a Nd-YAG laser, for example,the deep of penetration is approximately 50 mm.

The surface hit by the incident laser beam must be transparent in orderto allow the radiation to pass into the liquid cavity. This surface, theso-called window 7, can either be glass-clear or diffuse, the importantthing is that the absorption in this surface is low.

According to a preferred embodiment of the invention the end surface 6of the fiber is in optical contact with the window 7. The optical windowthen must have a good optical quality as also the original radiation ispassing through this window. By means of the optical contact between thewindow and the end surface of the fiber, basically all reflection lossescan be eliminated in the interface zone, see FIG. 2. Optical contact canbe achieved by means of fusing the fiber and the window together likeillustrated in the above-mentioned SE 93.01100-5 for the rod fusedtogether with the end surface of the fiber. The window 7 should becomparatively thick in order to allow an anti-reflex coating on thesurface.

In addition to the radiation which falls completely outside the fiber,also such radiation that has passed into the cladding 9 should bedirected out into the surrounding coolant. This can be achieved by meansof a so-called mode stripper 10 arranged on the fiber as illustrated inFIG. 3. The mode-stripper could be a glass-capillary of the type whichhas been previously illustrated in SE 83.07140-7 and SE 93.01100-5 andin which the glass-capillary is bonded to the envelope surface of thecladding and thus removing any radiation from the cladding.

Alternatively, the mode-stripping could be achieved by means of aroughening of the envelope surface of the fiber. Such roughening ispreviously known per se, see U.S. Pat. No. 4,575,181. Due to theroughening, the radiation in the cladding will be directed out from thecladding into the coolant where it is absorbed.

In the embodiment described so far the window and the fiber were fusedtogether in order to obtain a good optical contact. Another way toobtain a good optical contact is to press the fiber and the windowagainst each other. Also in such case the losses in the contact zone arenegligible.

Instead of applying the end surface of the fiber against a window 7 anoptical disc 11 having a central bore can be used, see FIG. 4. The disc11 can be made as a transparent, non-absorbing limiting surface (frontwall) for the coolant in the same way as the window 7, but with acentral opening into which the fiber end has been inserted. The disc isarranged with an appropriate sealing against the envelope surface of thefiber. The disc need not to be of the best optical quality as the majorpart of the radiation 2 does not pass through the disc but directly intoend surface of the fiber. The aperture disc can be either glass-clear ormat finished.

In one of the embodiments the fiber 3 is surrounded by a capillary tube12 made of a transparent material, for example quartz glass, so that theenvelope surface of the capillary tube is in contact with the coolant.In FIG. 5 there are three examples of capillary tube applications. InFIG. 5a the capillary tube 12 extends up to and is sealed against theinner surface of the window 7. In the example in FIG. 5b the fiber 3extends through the front window (disc) 11. Even in this case thecapillary tube is sealed against the inner surface of the disc 11. Inthe third example, which is illustrated in FIG. 5c, also the capillarytube 12 itself extends through the front wall 13. In this case it is notnecessary that the front wall is transparent as the radiation incidentoutside the end surface 14 of the capillary tube is negligible.

The object of the capillary tube is to provide an extra protecting,non-absorbing casing of the fiber. As already mentioned the capillarytube is sealed against both the front and rear walls of the cavity sothat the coolant is closed within the annular spacing formed between theenvelope surface of the capillary tube and the cylindrical outer wall ofthe cavity and will not come into contact with the envelope surface ofthe fiber itself. Contrary to the glass capillary used as a modestripper this capillary tube need not be fixed by cement on the fiber.In case a capillary tube is used the mode stripping is made byroughening the surface of the cladding.

The invention is not limited to the illustrated examples but can bevaried within the scope of the accompanying claims.

What is claimed is:
 1. An optical fiber cable of the type comprising afiber with a central core and a surrounding cladding said fiber cablebeing made for transmitting optical high-power, specifically powerexceeding 1 kW, and at least one of the fiber ends having cooling meansfor optical power loss comprising a cavity with a flowing, absorbingcoolant surrounding the envelope surface of said fiber end so thatincident optical radiation falling outside the fiber comes into and isabsorbed at least partially by said coolant and wherein the limitingwall surfaces of the cavity comprise a forward, at least partiallynon-absorbing surface through which the radiation is entered, while thelimiting wall surfaces are arranged to be directly cooled by saidflowing coolant to avoid any uncontrolled heating of said surfaces dueto absorbed radiation and wherein the forward limiting wall surfacecomprises a transparent window and the end surface of the optical fiberis brought into optical contact with said window.
 2. Optical fiber cableaccording to claim 1 wherein said coolant is a liquid coolant,preferably water.
 3. Optical fiber cable according to claim 2 whereinthe fiber is in direct contact with the liquid coolant.
 4. Optical fibercable according to claim 2 wherein the fiber is surrounded by atransparent tube which is in direct contact with the liquid coolant. 5.Optical fiber cable according to claim 2 wherein the limiting wallsurfaces of the cavity filled with the liquid coolant a substantiallycylindrical limiting wall surface extending coaxially in thelongitudinal direction of the fiber and a rear, limiting wall surfacethrough which inlet- and outlet pipes are arranged for the liquidcoolant.
 6. Optical fiber cable according to claim 1 wherein the endsurface of the optical fiber is fused together with the window. 7.Optical fiber cable according to claim 1 wherein the end surface of theoptical fiber has been pressed against the window for a good opticalcontact between the window and the fiber.